1. Field of the Invention
The present invention relates to fluorine-containing synthetic quartz glass having a high transmittance to radiation with a wavelength of up to 400 nm, and particularly to radiation in the vacuum ultraviolet region. The invention relates also to a process for producing such synthetic quartz glass.
2. Prior Art
The level of integration in semiconductor integrated circuits has continued to rise rapidly in recent years. Lithographic processes involved in the fabrication of semiconductor devices are making use of exposure light sources of increasingly short wavelengths. Today, lithography based on KrF excimer lasers, which have a wavelength of 248 nm, is becoming mainstream.
To achieve even higher levels of device integration, intensive efforts are being made to move on to shorter wavelength ArF excimer lasers (193 nm), and F2 excimer lasers with a wavelength of 157 nm show considerable promise for the not-too-distant future.
High-purity quartz glass having a high transmittance to light emitted by the light sources utilized and a low thermal expansion coefficient is used in lithography systems, both in light projection optics such as stepper lens and prisms and also in photomasks (reticles). Yet, prior-art quartz glass has a transmittance that gradually decreases in what is commonly referred to as the vacuum ultraviolet region below 200 nm, and absorption ceases altogether near 140 nm. Hence, its use in optical members such as photomasks has been regarded as problematic. The feasibility of using CaF2 single crystals, which have a higher transmittance than quartz glass, in lithographic system which employ exposure light sources emitting at wavelengths in the vacuum ultraviolet region has also been investigated. However, compared with quartz glass, CaF2 has a high thermal expansion coefficient, a low material strength, poor processability due in part to cleavage, and a high production cost. For these and other reasons, CaF2 appears to be poorly suited for use in lithography, and especially as a photomask material.
A pressing need thus exists for the development of quartz glass having a high transmittance to light in the vacuum ultraviolet region.
The decline in transmittance by quartz glass within the vacuum ultraviolet region is due primarily to absorption by structural defects in the glass. Structural defects which absorb in the vacuum ultraviolet region include primarily Si—Si bonds, Si—OH bonds, Si—O—O—Si bonds and Si—Cl bonds.
Therefore, the production of quartz glass which minimizes the formation of such structural defects and has a high transmittance in the vacuum ultraviolet region is vital to efforts for achieving the practical application of ArF and F2 excimer laser-based lithography.
In particular, Si—Si bonds, sometimes referred to as “oxygen deficiency defects,” have absorption bands at 163 nm and 245 nm, and thus present a problem with the use of F2 excimer lasers and also KrF excimer lasers as the exposure light source. Moreover, these defects lower the durability of quartz glass, both by creating, with excimer laser irradiation, defects called E′ centers which absorb at 215 nm (an effect known as “solarization”) and because of the fluctuations in refractive index triggered by the compaction that arises due to the absorbed energy.
Similarly, Si—O—O—Si bonds (oxygen surplus defects), which absorb at 177 nm, form non-bridging oxygen radicals under excimer laser irradiation, causing declines in the transmittance and durability of the quartz glass.
In the course of earlier research aimed at eliminating such structural defects, a method was proposed which called for producing a porous silica matrix by flame hydrolyzing a silica-forming raw material gas, then melting and vitrifying the porous silica matrix in a fluorine atmosphere.
This prior-art process reduces the number of the above-described structural defects in quartz glass and forms Si—F bonds. New absorption owing to the introduction of such bonds does not occur in the vacuum ultraviolet region at wavelengths of 140 nm and up. The reason is that Si—F bonds have a larger band gap than the Si—O bonds in quartz glass.
Moreover, because Si—F bonds have a large bond energy and are very stable, they do not form new structural defects such as E′ centers when exposed to excimer laser irradiation.
Accordingly, the formation of a high concentration of uniformly dispersed Si—F bonds within quartz glass should provide a quartz glass well-suited to use as an optical material for vacuum ultraviolet-related applications.
However, we have found it to be exceedingly difficult to produce good quartz glass having a desirable concentration and uniformity of Si—F bonds by conventional methods. That is, uniformly doping quartz glass with fluorine atoms to a high concentration of at least 2 wt % by prior-art methods is very difficult, yet lower concentrations fail to adequately curb the decrease in light transmittance. Also, high-concentration doping becomes increasingly difficult as the diameter of the porous silica matrix increases.
Moreover, vitrification of the porous silica matrix proceeds inward from the surface of the matrix. Thus, if fluorine doping is carried out at the same time, diffusion of the fluorine to the interior is inhibited by vitrification of the porous silica matrix. The inevitable result is the formation of quartz glass having a radial distribution in the fluorine atom concentration.
In addition, because the temperature of vitrification varies with the concentration of fluorine dopant, some unmelted portions may remain, resulting in incomplete vitrification. This tendency becomes increasingly acute at higher doping concentrations.
Even when vitrification is complete, uneven distribution of the fluorine atom concentration within the quartz glass causes non-uniformity in such optical properties as the transmittance and refractive index. As a result, although the quartz glass may have a high transmittance, it is poorly suited for use as a reticle substrate material because the transferred image ends up being partially out of focus. This tendency becomes increasingly pronounced as the absorption edge of quartz glass is approached, and is thus a problem which must be overcome to increase the precision of lithography.
When diffusing a fluorine compound gas into a porous silica matrix, factors believed to be effective for uniform doping include a low matrix bulk density, a small matrix diameter, and a long doping time. This is why methods have hitherto been used in which fluorine doping is carried out for an extended period of time on a low-bulk density matrix.
However, only a small amount of product can be obtained by vitrifying a low bulk density matrix. Moreover, a long doping time prolongs the production time and also increases the consumption of fluorine compound gas serving as the dopant. The resulting efficiency of production, including production costs, is much lower than that for ultraviolet-grade synthetic quartz glasses which do not contain fluorine.
Thus, to produce quartz glass having a high transmittance in the vacuum ultraviolet region, there has existed a need to develop a method for doping fluorine to a higher concentration and better uniformity than has hitherto been possible.